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I'm using 3B+ and there is something wierd. My pi connects to my router via wifi and cable at the same time without a problem. I can both ssh to the wlan ip and the eth ip address. However, when I turn off wifi, I can no longer connect to my pi while the cable is plugged in. It seems that wlan and eth are bounded somehow. If I unplug the cable, pi connects to the router via wifi perfectly without any issue.

Btw, I am using debian-transmission and shadowsocks if that matters. My purpose is to create a wifi hotspot from pi while it is connected to my router via cable. Then probably I can use proxy for the pi wifi hotspot. I found out that eth and wlan are somehow bounded in this procedure.

My question is: How do I allow eth while wlan is disabled?

Has anyone encoutered the same problem? Thanks in advance.

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I don't know details of your network configuration, particular with debian-transmission and shadowsocks. But in general there can be said something about connecting two interfaces to the same destination. It is a sort of failover. The RasPi uses only one connection of course, either eth0 or wlan0. What connection the kernel uses depends on the metric of the route. You can show it with ip route show. The route with the lowest metric is used first, then the next lowest one and so on.

It seems the route to your wifi network has the lowest metric so the kernel only uses it. Through wifi you can also reach the ip address from the eth0 interface because it's on the same device but not using the wired connection. And that's it. Bringing down wifi there is no connection anymore. Bring it up again and it works. The wired connection is never successful used.

There are some other issues with more than one interface and metric. Usually the kernel uses the next available interface but established TCP connections with the "old" interface are broken. To handle this you can use bonding. How to do it you can look at Howto migrate from networking to systemd-networkd with dynamic failover. It also explains the issue with the metric.

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